The Physics and Psychology of Colour - with Andrew Hanson

Andrew Hansen, a color and light measurement expert from the National Physical Laboratory (NPL), introduces the complex nature of color perception. He recounts the viral 'dress' debate, where the world argued over the colors of a dress in a photograph. Hansen's own experience with lighting a dress differently to reveal varying perceptions is shared. The importance of accurate color measurement in various fields such as branding, quality control, and legal and safety standards is emphasized. The NPL's role in physical measurements, including color and light, is highlighted, showcasing the significance of these measurements in everyday life and industry.

This paragraph delves into the human visual response to light and color. Hansen explains how light, with its wave properties, is split into its components, and how our eyes respond differently to various parts of the spectrum. He demonstrates this with a light bulb and a device that separates light into its spectral components. The sensitivity of the human eye to green light is discussed, along with the inability of our eyes to see ultraviolet and infrared light due to biological filters. The concept of observer metamerism is introduced, where individuals perceive colors differently under various lighting conditions.

Hansen outlines the three stages of color production: the light source, the object's interaction with light, and human perception. He uses a demo involving gaffer tape to illustrate how objects can appear differently under various light spectra. The importance of material properties in color reflection is emphasized, independent of the light source or viewing conditions. The paragraph also touches on the concepts of observer and sample metamerism, explaining how colors can appear different under various lighting conditions or to different viewers.

The speaker explores the complexities of color through phenomena like fluorescence, where an object can appear to emit light that is different from the light it receives. He uses a high-visibility jacket to demonstrate how an object can appear yellow under various light sources due to fluorescence. Afterimages are also discussed as a method to understand color perception, with an experiment involving staring at a pattern to induce the phenomenon. The paragraph explains how afterimages can reveal how the eye's sensors respond to different color wavelengths.

Hansen discusses the historical development of the 'standard observer,' a model based on the color perception of human subjects, which has become a universal reference for color measurement. He explains how color is perceived in three dimensions and how it's represented in two dimensions on a color chart. The use of this model in various technologies, from cameras to traffic lights, is highlighted. The paragraph also covers how color blindness can be plotted on the same color chart, showing how different types of color vision deficiencies affect the perception of hues.

This section describes advanced methods for measuring color, such as using a spectrophotometer to capture and compare the spectrum of light reflected by an object against a standard light source. Hansen also mentions a simpler method used in digital cameras, which measures the red, green, and blue components of light to approximate color. The complexity of perceived color beyond the RGB model is acknowledged, emphasizing that human color perception is influenced by more than just the primary colors.

Hansen transitions from physics to biology and psychology in exploring color perception. He discusses the mosaic of red, green, and blue sensitive cells in the retina and how their distribution varies by individual. The nonlinear response of human vision to light is highlighted, with experiments showing that equal steps of light intensity do not appear as equal increases in brightness. The effects of aging on color perception, such as the yellowing of the eye's lens, are also covered, demonstrating how different people can perceive the same color scene differently.

The final paragraph examines the dynamic and contextual nature of color perception. It discusses how the brain uses the whitest part of a scene as a reference for color constancy, adjusting the perceived color of objects relative to this reference. The paragraph also touches on the effects of surrounding colors on perception, using examples of identical colors appearing different based on their context. The complexity of color perception is reiterated, with a final demonstration showing how identical colors can appear different based on size and surrounding colors.